This R21 application responds to PAR-14-053. Research examining the theoretical models proposed by treatment developers is a valuable approach to enhancing the impact of existing evidence-based behavioral addictions treatments (Magill & Longabuagh, 2013). The intent is to empirically-derive the causal processes that are responsible for treatment benefit, and the result is identification of: 1) key therapeutic behavior, 2) skill proficiency thresholds, 3) key proximal client outcomes, and 4) the conditions under which these principles are most operative (i.e., conditional process models). Motivational interviewing (MI) is arguably the most important behavioral treatment upon which to conduct this kind of research. Recently, Magill and colleagues (Magill et al., 2013; MS under revise/resubmission) conducted a meta-analysis of studies testing specific pathways of the Technical Model of MI Efficacy as proposed by Miller and Rose (2009). While this research contributes to an emerging picture of how MI affects behavior change, an additional, more comprehensive meta-analytic study is needed to test the full theoretical framework, to identify conclusively which causal pathways have support, under what conditions, and which, if any, require major theoretical revision. The proposed project will address this aim not only with a larger sample of primary studies, but also with a database comprised of raw, rather than published and available, data. The Project will first compile a comprehensive dataset of bivariate correlations for fulfillment of all scientific aims. Next, it will test the Technical, Relational, nd a series of conditional process models (e.g., by client and treatment implementation factors) of MI efficacy at an aggregate, meta-analytic level. Finally, this large-scale, primary study dataset wil enable an exploratory, psychometric assessment of observationally-rated, within-session summary scores as well as individual therapist and client behavior codes. The breadth of findings from this study will inform the following three key areas of the MI evidence base: a) clinical guidelines for MI implementation, b) knowledge on MI causal theory, and c) empirically-derived measurement guidelines for future MI process research.